Epicyclic Gearbox for Counter-Rotating Fans
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Solution Overview
Problem
Existing epicyclic gearboxes in aircraft gas turbine engines with counter-rotating fans face challenges in reducing planet bearing loads, gear misalignment, and heat generation, which affect reliability, weight, and efficiency.
Innovation Solution
The design includes axially aligned or adjacent forward and aft roller bearings with a specific arrangement of output teeth and gears, supported by conical carrier frames, which reduces bearing loads and misalignment through optimized gear meshing and helical tooth configurations, and is integrated into an epicyclic gearbox to drive counter-rotatable fan stages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional epicyclic gearbox design is used, then counter-rotating fan function is achieved, but planet bearing loads are excessive
Solution Approach 1:
The planetary gear is divided into two separate gear stages (first planetary gear stage and second planetary gear stage) with different function. The first stage handles power transmission while the second stage is dedicated to counter-rotation. This segmentation allows each stage to be optimized independently, reducing the bearing loads on planetary gears.
Solution Approach 2:
A dedicated counter-rotation idler gear is introduced as an intermediary element between the two planetary gear stages. This idler gear specifically handles the counter-rotation function, allowing the main planetary gears to focus on power transmission and reducing their bearing loads significantly.
2Reliability
If larger bearings are used to handle high loads, then reliability improves, but weight increases
Solution Approach 1:
By segmenting the gearbox into two planetary gear stages with a dedicated counter-rotation idler, the load on each bearing is reduced. This allows the use of smaller, lighter bearings while maintaining or improving reliability, as each bearing only needs to handle a portion of the total load rather than the full load.
3Manufacturing precision
If gear tooth shape is modified to compensate for deflection, then alignment improves, but manufacturing complexity increases
Solution Approach 1:
The counter-rotation function is extracted from the main planetary gear mechanism and handled by a dedicated idler gear. This separation eliminates the need to modify gear tooth shapes to compensate for deflection, as the idler gear provides the counter-rotation function without interfering with the main power transmission path.
4Strength
If helical gear teeth are used, then gear durability improves, but heat generation increases
Solution Approach 1:
By dividing the power transmission into two separate planetary gear stages, the load and heat generation at each stage is reduced compared to a single stage. This allows the use of helical gears for improved durability while the distributed load configuration helps manage heat generation through better heat dissipation across multiple contact points.
Data Source
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AI summary
An epicyclic gear train (40) includes planetary gears (74) rotatably mounted on spindles (75) supported by an annular carrier (72) and including axially spaced apart forward and aft sets of output teeth (94, 96) extending radially outwardly from a planetary gear hub (92) and axially spaced apart forward and aft roller bearings (111, 112) disposed between planetary gears (74) and spindles (75). The forward and aft roller bearings (111, 112) are axially aligned with or adjacent to spaced apart forward set of output teeth (94) and input gear (104) respectively. A ring gear (100) meshes with forward set of output teeth (94) and an external gear (102) meshes with aft set of output teeth (96). An input gear (104) fixedly attached to hub (92) aft of aft set of output teeth (96) and engaged with a sun gear (108). The output teeth (94, 96), input gear (104), ring gear (100), external gear (102), and sun gear (108) may all be helical. A turbofan gas turbine engine (10) may include counter-rotatable first and second fan stages (60, 62) driven by a low pressure turbine (32) through the gear train (40).